The present invention relates generally to techniques and apparatus for activating implanted battery-operated medical devices, such as neurostimulators for treating or controlling medical, psychiatric or neurological disorders by application of modulating electrical signals to a selected nerve or nerves of the patient.
Extra-physiologic electrical stimulation of the vagus nerve for treatment of epilepsy and various forms of involuntary movement disorders is disclosed in U.S. Pat. No. 4,702,254 to J. Zabara (referred to herein as "the '254 patent"). An implantable neurocybernetic prosthesis (NCP) utilizes neurocybernetic spectral discrimination by tuning the external current of the NCP generator to the electrochemical properties of a specific group of inhibitory nerves that affect the reticular system of the brain. These nerves are embedded within a bundle of other nerves, and are selectively activated directly or indirectly by the tuning of the NCP to augment states of brain neural discharge to control convulsions or seizures. According to the patent, the spectral discrimination analysis dictates that certain electrical parameters of the NCP pulse generator be selected based on the electrochemical properties of the nerves desired to be activated.
An improved implantable neurostimulator device is disclosed in copending U.S. patent application Ser. No. 07/434,985, filed Nov. 10, 1989, now U.S. Pat. No. 5,154,172, issued Oct. 13, 1992 to Reese S. Terry, Jr., et al. (referred to herein as "the '172 patent), assigned to the same assignee as the instant application. The disclosure of the '172 patent is incorporated by reference herein. For the sake of convenience, a block diagram of the stimulus generator of a neurostimulator of the type disclosed in the '172 patent is illustrated in FIG. 1, and further details of location of an implantable version of the device and the associated lead/electrode system are shown in FIG. 2. The implanted device communicates with a programmer and/or monitor external to the patient's body by means of asynchronous serial communication, to control and indicate device states.
Stimulus generator 10 is implanted in the body of a patient 30 in a surgically-formed pocket immediately beneath the skin in the chest (FIG. 2). Housing 21 is hermetically sealed and composed of a material biologically compatible with the fluids and tissue of the patient's body. The neurostimulator also includes implantable stimulating electrodes 25 together with a lead system 22 for applying the output signal of the stimulus generator to a selected nerve such as the patient's vagus nerve 27. Components external to the patient's body include a programming wand 33 for telemetry of parameter changes to the stimulus generator and monitoring signals from the generator, and a computer 35 and associated software for adjustment of parameters and control of communication between the generator, the programming wand and the computer.
The stimulus generator includes a battery 12, such as a lithium thionyl chloride cell, having terminals connected to the input of a voltage regulator 13. The regulator smoothes the battery output and supplies power to logic and control section 15, which includes a microprocessor and controls the programmable functions of the device, such as current or voltage, frequency, pulse width, on-time and off-time of the output pulses generated by the genrator. The programmability allows the output pulse signal to be selectively tailored for modulating the electrical activity of the vagus nerve to produce the treatment regimen applicable to the disorder. Timing signals for the logic and control functions of the generator are provided by a crystal oscillator 16. A magnetically-actuated reed switch 14 provides the capability for patient activation of the device.
Built-in antenna 17 enables communication between the implanted stimulus generator and the external electronics (including both programming and monitoring devices) to permit the device to receive programming signals for parameter changes, and to transmit telemetry information, from and to the programming wand. Once the system is programmed, it operates continuously at the programmed settings until they are reprogrammed (by the attending physician) by means of the external computer and the programming wand.
It is important to conserve energy in any battery operated device implanted for medical treatment of a disorder. To that end, a power down circuit 18 may be electrically connected to reed switch 14 and logic/control circuit 15 and timed by the clock pulses from the crystal oscillator 16 to reduce power to the microprocessor of section 15 and/or to the oscillator to a point at which the device is essentially in a sleep state but sufficiently alert to be awakened on command. The power down mode or sleep state may be initiated automatically within a timed interval after the device has been activated to generate its programmed stimulating output signal. Alternatively, the device may stay in a reduced power state until the microprocessor is awakened by manual activation of the device by the patient.
Logic/control section 15 of the stimulus generator 10 controls an output circuit 19 which generates programmed signal levels. The programmed output signal of section 19 is fed, via an electrical connector 20 on the generator case (housing) 21, to the lead assembly 22 which is connected at its distal end to the stimulating electrodes (FIG. 2). The parameters of the stimulating signal of the implanted device are calibrated by telemetry (via the programming wand 33) according to the patient's needs, and programmed into the microprocessor for delivery of treatment upon activation of the stimulus generator.
FIG. 2 illustrates the location of generator 10 in the patient's chest with nerve electrode array 25 and associated lead 22 implanted in the patient's neck. The lead is attached at its proximal end to connector 20 of housing 21. Electrode array 25 is a bipolar stimulating electrode, for example of the type described in U.S. Pat. No. 4,573,481 to Bullara.
The implanted NCP of the '254 patent or neurostimulator disclosed in the '172 patent is activated manually or automatically to provide treatment for the duration of the seizure. The patient can manually activate the device by positioning a magnet over the implant site to actuate the reed switch at onset of the seizure. Automatic activation is triggered upon detection of instantaneous changes in certain state (EEG) parameters immediately before or at onset of a seizure. Also, a prophylactic or preventive mode may be employed in which the implanted device is activated periodically to reduce the occurrence and/or the intensity of the seizures.
It is a principal object of the present invention to provide improvements in techniques for manual and automatic activation of an implanted neurostimulator.